Scientists have long drawn a connection between population growth and increased greenhouse gas emissions, but previous research has not focused on demographic shifts that are likely to go along with the increase in the number of people.

World population is expected to be generally older and more urban by century's end, and more people are likely to live in smaller households rather than in extended families.

But how many people might there be?

The researchers considered three scenarios: a continuation of current trends, which would yield a 2-billion-person increase by 2050; a slower growth path which could mean about 1 billion more people; and a faster growth path, which could see a population rise of as much as 3 billion by 2050.

That would mean about 9 billion people living on Earth, compared to more than 6 billion now.

A slower-growth path could cut emissions by 16% to 29% of the amount needed to keep global temperatures from causing serious effects, the researchers say. And an aging population with lower participation in the workforce could cut emissions by as much as 20% in some industrialised countries.

Urbanisation

Generally speaking, the more people there are, the more fossil fuel they use, causing more greenhouse gas emissions.

But those who live in rural areas, particularly in developing countries, use more biomass as fuel instead of fossil fuels like coal and oil, says lead author Brian O'Neill of the US National Center for Atmospheric Research.

Therefore, a big move from country to city living is likely to mean greater fossil fuel use, especially in the developing world.

Even if city-dwellers have relatively smaller carbon footprints - living in smaller spaces, using public transit and less fossil fuel per person - an influx of country people into cities is likely to make greenhouse emissions rise.

Another effect of urbanisation is that urban workers tend to contribute more to economic growth than rural workers do.

"That's not because they work harder or longer hours," says O'Neill.

"It's because they're in sectors of the economy that drive economic growth more."

As a result, he says, the whole economy of the country grows faster and overall demand for energy rises, driving emissions up, as much as 25% in some developing countries.

The trend toward urbanisation could have a noticeable impact on energy demand, especially in Asia, says O'Neill.

"I think it's possible ... that we're underestimating potential growth rates in energy demand in regions of the world that may urbanise very quickly over the next 20 to 30 years," he says.